The use of transponders, in particular of RFID transponders (radio frequency identification transponders), has already become widely accepted in countless areas of life and work. Due to the property of RFID transponders, in the following also referred to simply as transponders, of recording, processing and rereleasing information contactlessly via electromagnetic fields, they can contribute in a particularly advantageous manner to reducing what are referred to as media disruptions, that is to say gaps between the real physical world and the digital world of information processing. In particular, their use focuses on automatic identification methods that contribute towards improving business management or technical processes, safety-related procedures and also general identification processes. Typically, such transponders are used for real-time identification in supply chains and in logistics processes of commercial enterprises, the pharmaceutical industry, the automotive industry and other manufacturing companies and they support the control and progress of specific production and logistics operations. Transponders are typically attached to packages, pallets and containers so that they are easily accessible and can quickly be activated.
RFID technology is typically based on the fact that serial numbers of codes are stored on a microchip integrated in a transponder whereby the microchip may be used both as an electronic data memory and also, for example, for signal processing. The serial number or codes may be read, for example, via wireless communication by means of magnetic fields. First of all a transponder antenna is activated by resonance by means of an external magnetic field. The transponder converts the signal received in a predetermined manner and then re-emits a magnetic field, or modifies the signal beamed in such a way that external resonators in a reader can detect this field and as a result allow information to be read. In this case the coupling of reader and transponder takes place in a manner comparable to the principle of the loosely coupled transformer. In this case data transmission by means of a magnetic field typically takes place in the HF range. In addition, state of the art RFID technology also allows a plurality of RFID transponders to be read simultaneously and completely automatically without there having to be a direct line of sight between a reader and the RFID transponder. Similarly, the RFID technology permits use in adverse environments by embedding transponders, for example, in objects and these thus not being directly affected by immediate environmental impacts. Compared to barcode scanners, for example, reading over a larger distance is also possible whereby in addition information may also be stored and modified on an RFID transponder thus enabling a very flexible and dynamic identification process.
If, however, such RFID transponders are used as a substrate on metallically conductive surfaces, the functionality of these devices is sometimes reduced dramatically since, because of the magnetic fields used for communication between transponder and reader, eddy currents may be induced in the metallic material and also sometimes in the antenna structure, and as a result electric excitation of the transponder antenna cannot be carried out with sufficient energy. By beaming in a magnetic alternating field, electrical eddy currents are induced in the metal surfaces such that according to Lenz's rule they for their own part induce an alternating magnetic field which counteracts the magnetic field beamed in. Consequently, the resulting magnetic field on the metallic surface is weakened to such an extent that adequate energy supply of the transponder and data transfer become impossible. In addition, due to the metallic surface, in the event of direct contact with the electrically conductive transponder, or even by simply approaching it, there is a clear de-tuning of the transponder's resonant frequency, as a result of which it either becomes necessary to retune the reader to the altered frequency or other precautions must be taken in order to enable communication between reader and transponder at customary permanently pre-set resonant frequencies.
In order to find a remedy for such problems, mechanical spacers among other things are used in conjunction with transponders on a metallic surface, which attach the transponders in a position elevated above the metallic material. Although this ensures the transponder's functional capability, the construction may sometimes represent a significant limitation due to the resulting overall height of transponder and spacer. Gaps of a few centimeters are not unusual for such spacers. Moreover, the production costs for such spacers may also be far above the costs for the actual transponder and thus make this technology appear unattractive for many processes where high quantities are used.
Beyond this, thin plastic-bonded ferrite films are also used for magnetic field screening. The manufacture of such a ferrite film, however, places high demands on the homogeneity of the composite material and therefore results in relatively high costs for manufacture. After lamination of this typically viscoplastic, brittle material made of ferrite and plastic, a stiff and barely flexible tag is created with an overall height of typically at least one millimeter. The device can only be attached to flat surfaces due to the stiffness. High manufacturing costs and the thickness of the entire device additionally restrict the application range of this magnetic field screening.
DE 100 17 142 A1 describes a self-adhesive data exchange tag which provides for a transmitter and receiver device as well as a transponder antenna, whereby an erectable section is also provided, using which the antenna can be brought into a position perpendicular to a metallic surface. Although the data exchange on metallic surfaces is improved with this positioning of the antenna, nevertheless the disadvantage which arises is again a large average distance between the antenna and the metallic surface and low mechanical stability of the data exchange tag.
EP 1 594 082 A1 describes a configuration and a method for attaching a transponder element to an object such as a metallic surface, whereby a tag furnished with expansion or shrink film is provided, said tag being attached between the surface or substrate and the transponder. Subsequently, the expansion or shrink film is activated by supplying heat or UV light, whereby the gap between transponder and surface can be adjusted advantageously such that, for example, the result is a reduction of eddy currents on a metallic surface and improved reading of the transponder. The device described is again characterised, however, by a relatively large overall height, and similarly requires that there are no mechanical forces acting on the tag. In addition, due to activation by means of heat or UV light there may be adverse effects on and damage to the object on which the tag has been attached.
DE 101 49 126 A1 describes a device for screening a transponder and a method for manufacturing corresponding screening. Screening is facilitated by using small particles, preferably 300 μm×50 μm×10 μm in size, which are embedded in a liquid matrix and aligned in a constant magnetic field. The alignment is carried out in such a manner that the particles run parallel to a magnetic field induced by the transponder. The screening is preferably applied to a surface using printing technology, whereby curing of the printing structures takes place subsequently. The overall process is complex and results in a brittle screen with limited flexibility. Due to the printing process, the application is normally reduced to flat surfaces or makes it necessary to use a relatively complicated printing method. The constantly parallel alignment of the particles to the induced magnetic field lines of the transponder requires expensive magnetisation units to generate complex magnetic fields in the case of complex antenna geometries. Moreover, high-precision alignment during the lamination process is required for bonding of the transponder and screen in order to achieve parallelism between the magnetically induced transponder field and the aligned particles, which makes mass production considerably more difficult and consequently leads to higher costs for an end product.